1. Field of the Invention
The present invention relates to an apparatus and a method for coating a sleeve body with a single or a multitude of uniform layers of a coating formulation.
More specifically the invention is related to a coating device and method of coating wherein an irradiation stage is moveable with a coating stage. The irradiation stage at least partially cures the coated layer applied by the coating stage.
2. Description of the Related Art
Flexography is today one of the most important processes for printing. It is a method that is commonly used for high-volume runs. Flexography is employed for printing on a variety of substrates such as paper, paperboard stock, corrugated board, films, foils and laminates. Packaging foils and grocery bags are prominent examples. Coarse surfaces and stretch films can be economically printed only be means of flexography, which indeed makes it very appropriate for packaging material printing.
It uses a rubber printing plate or a flexible photopolymer plate that carries the printing image in relief. The ink delivery system for flexography is achieved via an “anilox” engraved transfer roll.
Analogue flexographic printing plates are prepared from printing plate precursors that include a photosensitive layer on a support or substrate. The photosensitive layer includes ethylenically unsaturated monomers or oligomers, a photo-initiator and an elastomeric binder. The support preferably is a polymeric foil such as PET or a thin metallic plate. Imagewise crosslinking of the photosensitive layer by exposure to ultraviolet and/or visible radiation provides a negative working printing plate precursor which after development with a suitable developer (aqueous, solvent or heat development) leaves a printing relief, that can be used for flexographic printing.
Imaging of the photosensitive layer of the printing plate precursor with ultraviolet and/or visible radiation is typically carried out through a mask, which has clear and opaque regions. Crosslinking takes place in the regions of the photosensitive layer under the clear regions of the mask but does not occur in the regions of the photosensitive layer under the opaque regions of the mask. The mask is usually a photographic negative of the desired printed image. Flexographic printing plate making according to the above described process has the disadvantage that the production of a mask is time consuming and that the dimensional stability of these masks with changing environmental temperature or humidity is unsatisfactory for high quality printing and color registration. Moreover, the use of separate masks in flexographic printing plate production means additional consumables and chemistry, with a negative impact on economy and ecology aspects of the production process, which are far more a concern than the additional time required to make the masks. As a matter of fact, in most cases plate exposure and plate development may turn out to be more time consuming than mask making.
Direct digital imaging using laser recording of printing plate precursors, which eliminates the making of a separate film mask, is becoming increasingly important in the printing industry. The flexographic plate is made laser-sensitive by providing e.g. a thin opaque IR-sensitive layer to the photopolymerizable layer of the flexographic plate. Such a plate is sometimes called a “digital” flexo plate. The thickness of such IR-ablative layers is usually just a few μm. The IR-ablative layer is inscribed imagewise using an IR laser, i.e. the parts in which the laser beam is incident on are ablated, i.e. removed. The actual printing relief is produced in the conventional manner: exposure is effected with actinic light (UV, visible) through the mask produced, being imagewise opaque to the crosslinking inducing light, and the relief layer is thus selectively crosslinked. Development can be effected with an organic solvent, water or heat removing the photosensitive material from the unexposed parts of the relief-forming layer and the residues of the IR-ablative layer, either one by one using different developing steps or simultaneously using one developing step.
This method still requires a developing step as in the case of previous methods and hence the improvement in efficiency for producing flexo printing plates is limited.
In the direct laser engraving technique for the production of flexographic printing plates, a relief suitable for printing is engraved directly into a layer suitable for this purpose. By the action of laser radiation, layer components or their degradation products are removed in the form of hot gases, vapors, fumes, droplets or small particles and nonprinting indentations are thus produced. Engraving of rubber printing cylinders by means of lasers has been known since the late 60s of the last century. However, this technique has acquired broader commercial interest only in recent years with the advent of improved laser systems. The improvements in the laser systems include better focusing ability of the laser beam, higher power, multiple laser beam or laser source combinations and computer-controlled beam guidance. The actual engraving system includes efficient gas- and debris collecting systems. Direct laser engraving has several advantages over the conventional production of flexographic printing plates. A number of time-consuming process steps, such as the creation of a photographic negative mask or development and drying of the printing plate, can be dispensed with. Furthermore, the sidewall shape of the individual relief elements can be individually designed in the laser engraving technique.
Although photopolymeric printing elements are typically used in “flat” sheet form, there are particular applications and advantages to using printing elements in a continuous cylindrical form as a rubber or a polymer sleeve. Continuous printing forms provide improved registration accuracy and lower change-over-time on press. Furthermore, such continuous printing forms may be well-suited for mounting on laser exposure equipment, where it can replace the drum, or be mounted on the drum for exposure by a laser. Continuous printing forms have applications in the flexographic printing of continuous designs such as in wallpaper, decoration, gift wrapping paper and packaging.
Sleeves are made by coating, mold casting of an elastomeric layer onto a plastic or metallic cylinder, or winding a rubber ribbon around a plastic or metallic cylinder followed by a vulcanizing, grinding and polishing step. The forms preferable are seamless forms. As an alternative the elastomeric layer may be first applied on a flat support, which is then bent onto the carrier and bonded (cfr. NYLOFLEX® Infinity Technology from BASF).
At the print media fair DRUPA held in 2004 in Germany, Asahi Kasei showed a prototype of the Adless digital engraving technology for the production of endless photopolymer sleeves for digital engraving. It allows a liquid photopolymer material to be continually coated onto a sleeve/cylinder in a short time. The working principles of the technology are disclosed in published patent application JP 2003-241397 from Asai Kasei. The Adless system is based on a horizontal coating stage for applying a photopolymer coating onto a sleeve core. The gap between the sleeve core's peripheral surface and the coating stage is gradually increased, while rotating the sleeve core, to increase the thickness of the applied photopolymer coating layer. After coating, the coated material is cured through photo-polymerization or photo-crosslinking. A post-curing step of grinding and polishing the cured photopolymer layer is required to provide the necessary surface characteristics, such as evenness, to the photopolymer layer in order to make the sleeve suitable as a flexographic printing sleeve. The post-curing step is required a.o. because of photopolymer unevenness of the coating process and the presence of a polymer bulge at the location where the coating stage was withdrawn from the sleeve when stopping the coating process. The required grinding and polishing is a disadvantage of the Adless system. The large floor space required, seen the horizontal position of the coating system, is also a disadvantage.
Patent application publication JP 55-106567 from Canon discloses a vertical coating method and device for uniformly coating a setting paint onto a drum, fixing the paint onto the drum by providing a low-hardening energy and further hardening the fixed paint onto the drum by providing a high-hardening energy. The coating vessel and the equipment for providing the low- and high-hardening energy are fixedly mounted. The drum that is to be coated is attached to a lifting and lowering mechanism for firstly vertically immersing the drum into the coating vessel and subsequently lifting the drum out of the vessel and transporting the drum past an annular low-hardening energy device and then positioning the drum in front of a vertical high-hardening energy device. The disclosed coating device is suitable for the coating of drums limited in size (both length and diameter) because: (1) the length of the drum is limited to less than half the height of the equipment and less than the height of the vertical high-hardening energy device, and (2) the diameter of the drum is limited by the dimensions of coating vessel and the diameter of the annular low-hardening energy device.
U.S. Pat. No. 4,130,084 assigned to Stork Brabant B. V. discloses a vertical ring coater having an annular receptical containing a coating liquid and arranged coaxial with a vertically positioned thin walled perforated sleeve. A layer of coating liquid is applied on the periphery of the sleeve during vertical movement of the annular receptical along the vertically positioned sleeve. The layer of coating liquid is dried via heat energy provided via mounting flanges into the central part of the sleeve.
A need exists for a coating device with limited floor space requirements, suitable for making flexographic printing sleeves for direct laser engraving, without the need for grinding and polishing, that offers more flexibility towards types and sizes of sleeves and further reduces the access time and production cost of direct laser engraveable sleeves.